I believe what you are talking about are the "portlights" (nautical term for windows). Probably the most important dimension you need to find is the thickness of the cabin top (including liner) that the portlight frames need to span. You can specify whatever thickness of lexan or plastic you want.

But perhaps you are reusing your old frames and replacing the clear plastic (that older plexiglas gets crazed, doesn''t it?). Here''s a neat trick from the in-house physicist to get a reasonable estimate of the thickness of the plexiglas.

Tools needed: straight edge, scribe (carbide tipped hole center marker will do), or a glass cutting wheel, try square with a smooth, 45 degree surface (the kind that usually has a built in spirit level and a removeable steel ruler). Optional: Laser pointer.

Procedure: Find an old piece of mirrored glass or smooth, polished metal. Scribe a straight line on it (use a straight edge). Rub some dye or ink in the scribed line for better visibility. Next, use the straight edge and scribe to mark a thin straight line on your old plexiglas (you''re replacing it anyway, right?). Have an assistant hold the mirror with the scribed line on it inside the cabin. Lay the try square on the plexiglas and sight along its 45 degree surface (or tape your laser pointer to the surface, making sure the beam is parallel to the surface) until you can see the line you scribed in the plexiglass. Now have the assistant hold the mirror shard against the inside of the plexiglas and move it until the line on the mirror is precisely aligned with the one you scribed on the outside of the plexiglas. Have the assistant tape the mirror shard in place. Now turn the try square 45 degree surface around and sight along it in the other direction, and position it so that you are now lining up the line on the mirror with the reflection of the scribed line on the plexiglass. Take the straightedge and hold it next to these aligned marks, and scribe a second line that is aligned with the one on the mirror and the original one reflected in the mirror. If you did it right, you will now have a spacing between the two lines scribed in the outside surface of the plexiglas that is twice the thickness of the plexiglas. If you are doing it with a laser sight taped to the 45 degree surface, you can have the assistant look from the other direction to ensure the laser is hitting the original line scribed in the plexiglass. You should get a measurement that is close to some standard thickness (3/16, 1/4, or 6 mm, for example), since I believe glazing tends to be provided in standard thicknesses.

Of course, a much easier plan (since you are replacing them anyway) is to simply drill a small hole in the plexiglas and use a vernier caliper (back the hole up firmly with a piece of plastic or metal and use the depth measuring part of the caliper). You can tape over the small hole until your new windows are installed. This will probably give you a more accurate measurement than my fancy optical method above, unless you are using a laser sight.

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